In a vehicle having a continuously variable transmission, during deceleration before vehicle stop, by operating a transmission ratio up to a maximum transmission ratio by the time that the vehicle stops, starting performance at a subsequent re-start of the vehicle is ensured. However, in a case of the deceleration by hard braking during vehicle travelling, since a time required for the vehicle stop is short, it is not possible to operate the transmission ratio up to the maximum transmission ratio by the time that the vehicle stops. Further, if a wheel locks during the deceleration, even the operation to change the transmission ratio is not possible. Also after the vehicle stop, it is not possible to operate the transmission ratio as a matter of course.
Patent Document 1 discloses that in a vehicle provided with an auxiliary transmission having as a power transmission cut-off unit a clutch or a brake between the continuously variable transmission and a drive wheel, when the hard braking of the vehicle is detected, by disengaging the clutch or the brake of the auxiliary transmission and bringing power transmission between the auxiliary transmission and the drive wheel into a power transmission cut-off state, the changing operation of the transmission ratio becomes possible, then the transmission ratio is operated up to the maximum transmission ratio by the time that the vehicle re-starts afterwards. A technique of ensuring the starting performance, devised in this manner, is disclosed in the Patent Document 1.
Here, in the case of the technique of the Patent Document 1, when starting a control of the changing-operation of the transmission ratio to the maximum transmission ratio side with the power transmission brought in the cut-off state, as long as a throttle valve opening degree is 0 (zero) (a throttle valve is in a fully closed state), this control is performed until the transmission ratio reaches the maximum transmission ratio. Thus, during the carrying out of the transmission ratio control in this power transmission cut-off state, the power is not transmitted to the drive wheel. Therefore, if this control is performed when the vehicle stops on an upward slope (an uphill road), there is a risk that slide-down (or slip-down) of the vehicle will occur.
Further, in the case of the technique of the Patent Document 1, the power transmission cut-off is performed by pressure exhaust by exhausting working fluid from a working fluid supply oil passage that leads to the clutch and the brake. Thus, even when the transmission ratio control is completed and the power transmission is changed from the power transmission cut-off state to a power transmission allowable state, a time lag required for an engagement of the clutch or the brake occurs. For this reason, depending on a timing of the re-start, there is a possibility that a power transmission lag will occur. In order to reduce this power transmission lag, setting a line pressure to be high is effective. In this case, however, engagement shock of the clutch or the brake occurs, which is undesirable.
The present invention was made in view of the above technical problem. An object of the present invention is to provide a control device for the continuously variable transmission, which is capable of changing-operation of the transmission ratio of the continuously variable transmission up to the maximum transmission ratio (i.e. to a Lowest state) at the hard braking etc. of the vehicle even when the vehicle stops on the upward slope without the occurrence of the slide-down of the vehicle, and capable of suppressing the occurrence of the power transmission lag at the re-start without setting the line pressure to be high.